This application claims priority to Japanese patent application No. JPAP11-204743 filed on Jul. 19, 1999 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.
1. Field
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of effectively maintaining a charge potential.
2. Description of the Related Arts
Charging the surface of a photoconductive member is one of the basic and important processes performed in an image forming apparatus using an electrophotographic method, such as a copying machine, a facsimile machine, a printer, and so forth. Among a variety of techniques for consistently charging the surface of the photoconductive member, one exemplary techniques uses a charging member which is configured to make its surface contacting the photoconductive member so as to provide charges evenly to the surface of the photoconductive member. For example, a roller is suitably used as the charging member. Such a charging system using a charging roller has been widespread.
The charging system is often referred to as a main charging system in order to be distinguished from a charging system employed in a transfer mechanism. In particular, the charging system using a roller is referred to as a main charge roller system. This main charge roller system causes a discharge between the charge roller and the photoconductive member so as to provide an even charge on the surface of the photoconductive member. In the main charge roller system, whether the charge roller makes contact with the photoconductive member or not is not necessarily important factor to be considered but more important is to make a distance of a gap between these two members as small as possible. Accordingly, the main charge roller system usually has a charge roller in contact with the photoconductive member. A distance of 100 xcexcm or less of the gap between the charge roller and the photoconductive member is generally considered to be sufficient to make a consistent charge on the photoconductive member, provided that the charge roller is made of an optimum material and is applied with a charge of an optimum-voltage during an actual charging operation.
In the above-described main charge roller system, an efficiency of the charging operation which is obtained by dividing a charge potential by an applied voltage depends on a temperature of the charge roller. Accordingly, lower the environmental temperature, lower the charging efficiency. That is, in the main charge roller system using a constant-voltage control, the charge potential obtained under the constant-voltage control will be reduced when the charging efficiency is reduced. In this case, the main charge roller system will have problems of a reduction of image density and in controlling the image forming process in which the charge potential is used as a reference value for controlling the process.
Japanese Laid-Open Patent Publication No. JPAP4-6567(1992) describes a charging member which is heated to have a temperature in a range of from 35xc2x0 C. to 55xc2x0 C. However, this attempt has a drawback that the parts, such as the photoconductive member, other than the charge roller are also heated at the same time. More specifically, if the photoconductive member is heated to a relatively high temperature in a toner recycling system in which the toner remaining on the photoconductive member is collected and is returned back to a development unit, the toner remaining on the photoconductive member will also be heated and will likely change the character which causes various kinds of problems such as a toner blocking, a deterioration of toner agglomeration, and so on when it is reused in the development unit. In addition, the above-mentioned attempt has another drawback that the mechanism for heating the charge roller causes an increase of the manufacturing cost.
The present invention provides a novel charging apparatus. In one example, a novel charging apparatus includes a charge roller configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. This charge roller includes a plurality of roller layers. One of the plurality of roller layers has a resistance ratio with which a roller surface potential is raised by a charge movement. In this case, the resistance ratio has a value such that a time period in which a charge moves within the one of the plurality of roller layers is sufficiently smaller than a time period in which an arbitrary point of the surface of the charge roller moves across a discharge region formed between the charge roller and the photoconductive member.
The present invention further provides another charging apparatus including a charge roller configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. In this case, the charge roller includes a plurality of roller layers. One of the plurality of roller layers has a resistance ratio with which an excess current flow between a surface of the charge roller and the surface of the photoconductive member is prohibited. This resistance ratio has a value such that a time period in which a charge moves within the one of the plurality of roller layers is sufficiently greater than a time period in which an arbitrary point of the surface of the charge roller moves across a discharge region formed between the charge roller and the photoconductive member.
The present invention further provides another charging apparatus including a charge roller configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. In this case, the charge roller includes at least one roller layer having a resistance ratio which is defined in a way such that when a charge performance of the charge roller is obtained by calculating a relationship between the resistance ratio of the at least one roller layer and a charge potential of the surface of the photoconductive member using formulae of an Ohm""s law with consideration given to an advection member for a charge flow in two-dimensional directions within the at least one roller layer, a two-dimensional Poisson""s equation, and a Paschen""s discharge law, the calculated relationship includes two constant-potential regions in both which the charge potential stays at an approximate constant level relative to the resistance ratio and a resistance ratio in one of the two constant-potential region is selectee.
The present invention further provides another charging apparatus including a charge roller configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. In this case, the charge roller includes a plurality of roller layers. One of the plurality of roller layers has a resistance ratio with which a roller surface potential is raised by a charge movement. The resistance ratio is defined in a way such that when a charge performance of the one of the charge rollers is obtained by calculating a relationship between the resistance ratio of the one of the roller layers and a charge potential of the surface of the photoconductive member using formulae of an Ohm""s law with consideration given to an advection member for a charge flow in two-dimensional directions within the one of the roller layers, a two-dimensional Poisson""s equation, and a Paschen""s discharge law, the calculated relationship includes two constant-potential regions in both which the charge potential stays at an. approximate constant level relative to the resistance ratio and a resistance ratio in one of the two constant-potential region is selected.
The present invention further provides another charging apparatus including a charge roller configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. In this case, the charge roller includes a plurality of roller layers. One of the plurality of roller layers has a resistance ratio with which an excess current flow between a surface of the charge roller and the surface of the photoconductive member is prohibited. The resistance ratio is defined in a way such that when a charge performance of the charge roller is obtained by calculating a relationship between the resistance ratio of the one of the roller layers and a charge potential of the surface of the photoconductive member using formulae of an Ohm""s law with consideration given to an advection member for a charge flow in two-dimensional directions within the one of the roller layers, a two-dimensional Poisson""s equation, and a Paschen""s discharge law, the calculated relationship includes two constant-potential regions in both which the charge potential stays at an approximate constant level relative to the resistance ratio and a resistance ratio in one of the two constant-potential region is selected.
The present invention further provides another charging apparatus including a charge roller configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. In this case, the charge roller includes at least first and second roller layers. The first roller layer has a first resistance ratio with which a roller surface potential is raised by a charge movement. The second roller layer has a second resistance ratio with which an excess current flow between a surface of the charge roller and the surface of the photoconductive member is prohibited. The first resistance ratio has a value such that a charge movement time period in which a charge moves inside the first roller layer is sufficiently smaller than a process time period in which an arbitrary point of the surface of the charge roller moves across a discharge region formed between the charge roller and the photoconductive member. The second resistance ratio has a value such that the charge movement time period is sufficiently greater than the process time period.
The present invention further provides another charging apparatus including a charge member, a voltage applying circuit, a temperature detector, and a voltage controller. The charge member is configured to adjoin to a photoconductive member so as to provide a charge to a surface of the photoconductive member. The voltage applying circuit is configured to apply a voltage to the charge member. The temperature detector detects a temperature of or around the charge member and generating a signal variable in accordance with a detection result. The voltage controller is configured to calculate an electric field to obtain an optimum voltage to be applied to the charge member at a temperature of the charge member represented by the signal from the temperature detector so that the photoconductive member has an appropriate charge potential and to control the voltage applying circuit to generate and to apply the optimum voltage to the charge member.
In the above-mentioned charging apparatus, the charge member may be a charge roller including a plurality of roller layers and the voltage controller may include a memory for storing a data table having data representing a relationship between a temperature of the charge roller and each resistance ratio of the plurality of the roller layers. The voltage controller may obtain each resistance ratio of the plurality of the roller layers based on the signal from the temperature detector. The voltage controller may calculate the electric field based on the obtained each resistance ratio of the plurality of the roller layers to obtain the optimum voltage to be applied to the charge member so that the photoconductive member has the appropriate charge potential.
In the above-mentioned charging apparatus, the voltage controller may calculate the electric field to obtain the optimum voltage to be applied to the charge roller so that the photoconductive member has the appropriate charge potential with consideration given to variations of a resistance, a thickness, and a capacitor of each roller layer of the charge roller over time and variations of a resistance, a thickness, and a capacitor of the photoconductive member over time.
The voltage controller may calculate the electric field to obtain the optimum voltage to be applied to the charge roller so that the photoconductive member has the appropriate charge potential with consideration given to formulae of an Ohm""s law with consideration given to an advection member for a charge flow in two-dimensional directions within the one of the roller layers, a two-dimensional Poisson""s equation, and a Paschen""s discharge law.
Further, the present invention provides a novel image forming apparatus including a photoconductive member and a charging mechanism for charging the photoconductive member. The charging mechanism includes a charge roller configured to adjoin to the photoconductive member so as to provide a charge to a surface of the photoconductive member. In this case, the charge roller includes a plurality of roller layers. One of the plurality of roller layers has a resistance ratio with which a roller surface potential is raised by a charge movement. The resistance ratio has a value such that a time period in which a charge moves within the one of the plurality of roller layers is sufficiently smaller than a time period in which an arbitrary point of the surface of the charge roller moves across a discharge region formed between the charge roller and the photoconductive member.
Further, the present invention provides a novel method for manufacturing a charging apparatus. In one embodiment, a novel method for manufacturing a charging apparatus includes the steps of providing a photoconductive member and providing a charge roller with at least first and second roller layers. In this case, the first roller layer has a first resistance ratio with which a roller surface potential is raised by a charge movement. The second roller layer has a second resistance ratio with which an excess current flow between a surface of the charge roller and a surface of the photoconductive member is prohibited. The first resistance ratio has a value such that a charge movement time period in which a charge moves inside the first roller layer is sufficiently smaller than a process time period in which an arbitrary point of the surface of the charge roller moves across a discharge region formed between the charge roller and the photoconductive member. The second resistance ratio has a value such that the charge movement time period is sufficiently greater than the process time period. The novel method further includes a step of adjoining the charge roller to the photoconductive member so as to charge the surface of the photoconductive member.